Silicone rubbers are used in a wide variety of fields including electric equipment, automobile, construction, medical care, and food fields because of their excellent properties such as weather resistance, electric properties, a low compression set property, and heat resistance. Among silicone rubbers, a millable silicone rubber is widely used for producing rubber components because it can be mixed with a coloring agent and so on by a simple device such as a twin roll and can be molded by simple equipment such as a press, and especially because its shape can be retained even in an uncured state, it is widely used in extrusion molding of tubes, gaskets, and the like.
However, the unique tackiness that the surface of a cured silicone rubber has is likely to cause a problem of blocking of surfaces of its molded products. Forming minute irregularities on the surfaces can prevent the blocking, and in molded products formed using a mold, the blocking can be easily prevented by roughening the surface of the mold. However, in extrusion molding and coating which do not use a mold, it is difficult to control the surface state, and an improvement has been desired.
In the usage in the coating not using a mold, as an attempt to solve the blocking problem, Patent Reference 1 (JP-A No. 2006-225636), for example, describes a liquid silicone rubber coating agent composition in which gel-type silica having a 0.5 to 20 average particle size is used as a filler. However, even if the technique of this coating agent composition is applied to a composition for a silicone rubber molded product having an appropriate thickness as is produced by extrusion molding, the filler with a specific particle size compounded for preventing the blocking is not easily exposed to the surface, making it difficult to effectively solve the blocking problem. Further, in order to prevent the blocking by this method, an excessive amount of the filler needs to be compounded, which has a disadvantage that the silicone rubber loses its flexibility.
Further, a base compound of a conventional millable silicone rubber is produced by mixing a filler into a high-polymerization degree polyorganosiloxane in a crude rubber state, and it is a typical practice to compound low-viscosity disilanol or the like as process oil in order to facilitate the compounding of the filler.
Further, the aforesaid high-polymerization degree polyorganosiloxane in the crude rubber state is typically produced through ring-opening polymerization of low-molecular cyclic siloxane by an alkaline catalyst, but this reaction is an equilibration reaction, and a certain amount of the unreacted low molecular siloxane remains in the polymer. In recent years, there has been a demand for a reduction of the low molecular siloxane, but a difficulty in removing the low molecular siloxane from the base polymer in the crude rubber state necessitates high-temperature post-curing of a cured molded product, and inevitably causes a cost increase. Moreover, the high-temperature post-curing narrows a material selection width especially at the time of composite molding with a thermally vulnerable material such as thermoplastic resin.
To solve such problems, Patent Reference 2 (JP-A No. 2011-16977), for example, describes, a silicone rubber composition in which the content of a low-molecular siloxane with a 10 or less degree of polymerization in a polyorganosiloxane compounded in a base compound is reduced in order to prevent a contact fault of electronic components. However, in the medical care application and health care application, a reduction of all volatile siloxanes in addition to the low molecular siloxane with a 10 or less degree of polymerization has recently been required, and there is a need for a further improvement.